Electronic device and driving method thereof

ABSTRACT

A pixel is provided in which normal display of an image is possible even if a sustain period is shorter than an address period in a driving method combining digital gray scales and time gray scales, and in which operation can be compensated by changing the electric potential of a signal line even for a case in which the EL driver transistor becomes normally on due to deterioration. One of a source region and a dram region of an erasure TFT is connected to an electric current supply line, and the remaining one of the source region and the drain region is connected to a gate signal line. It is possible to change the voltage between a gate and a source of an EL driver TFT by changing the electric potential of the gate signal line so that the EL driver TFT is placed in a non-conducting state with certainty with this structure, even for cases in which the EL driver TFT becomes normally on due to a shift in the value of its threshold voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/532,307, filed Sep. 15, 2006, now allowed which is a continuation ofU.S. application Ser. No. 10/619,053, filed Jul. 15, 2003, now U.S. Pat.No. 7,113,155, which is a divisional of U.S. application Ser. No.09/841,098, filed Apr. 25, 2001, now U.S. Pat. No. 6,611,108, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2000-125993 on Apr. 26, 2000, all of which is incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic device structure. Inparticular, the present invention relates to an active matrix electronicdevice having a thin film transistor (TFT) formed on an insulating body,and to a method of driving an active matrix electronic device.

2. Description of the Related Art

EL displays (also referred to as electroluminescence displays) have beengathering attention in recent years as flat panel displays, which aresubstitutes for LCDs (liquid crystal displays), and research into suchdisplays is proceeding apace.

LCDs can roughly be divided into two types of driving methods. One is apassive matrix type used in an LCD such as an STN-LCD, and the other isan active matrix type used in an LCD such as a TFT-LCD. EL displays canalso be similarly broken down roughly into two types. One is a passivematrix type, and the other is an active matrix type.

For the passive matrix type, wirings which become electrodes arearranged in portions above and below EL elements (also referred to aselectroluminescence elements). Voltages are applied to the wirings inorder, and the EL elements turn on due to the flow of an electriccurrent. On the other hand, each pixel has a thin film transistor withthe active matrix type, and a signal can be stored within each pixel.

A schematic diagram of an active EL display device is shown in FIGS. 13Aand 13B. FIG. 13A is a schematic diagram of an entire circuit, and asubstrate 1350 has a pixel portion 1353 in its center. Gate signal linedriver circuits 1352 for controlling gate signal lines are arranged tothe left and right of the pixel portion. The arrangement may also be ononly one side, left or right, but considering such issues as operationalefficiency and reliability, it is preferable to use both positions asshown in FIG. 13A. A source signal line driver circuit 1351 forcontrolling source signal lines is arranged above the pixel portion. Onepixel portion circuit in the pixel portion 1353 of FIG. 13A is shown inFIG. 13B. Reference numeral 1301 denotes a TFT which functions as aswitching element during write in to the pixel (hereafter referred to asa switching TFT) in FIG. 13B. Reference numeral 1302 denotes a TFT whichfunctions as an element (electric current control element) forcontrolling electric current supplied to EL elements 1303 (hereafterreferred to as an EL driver TFT). The EL driver TFT 1302 is arrangedbetween an anode of the EL element 1303 and an electric current supplyline 1307 in FIG. 13B. It is also possible, as a separate structuringmethod, to arrange the EL driver TFT 1302 between a cathode of the ELelement 1303 and a cathode electrode 1308. However, from the fact thatit is good for TFT operation to have a source region connected toground, and from limitations on the manufacture of the EL elements 1303,a method in which a p-channel TFT is used in the EL driver TFT 1302 andis arranged between the anode of the EL element 1303 and the electriccurrent supply line 1307 is generally seen and often employed. Referencenumeral 1304 denotes a storage capacitor for storing a signal (voltage)input from a source signal line 1306. One terminal of the storagecapacitor 1304 is connected to the electric current supply line 1307 inFIG. 13B, but a specialized wiring may also be used. A gate electrode ofthe switching TFT 1301 is connected to a gate signal line 1305, and asource region of the switching TFT 1301 is connected to the sourcesignal line 1306. Further, a drain region of the EL driver TFT 1302 isconnected to an anode 1309 of the EL element 1303, and a source regionof the EL driver TFT 1302 is connected to the electric current supplyline 1307.

The EL element has a layer (hereafter referred to as an EL layer)containing an organic compound in which electroluminescence(luminescence generated by the addition of an electric field) isobtained, an anode, and a cathode. As to the luminescence in the organiccompound, there is emission of light when returning to a ground statefrom a singlet excitation state (fluorescence), and emission of lightwhen returning to a ground state from a triplet excitation state(phosphorescence), and the electronic device of the present inventionmay use both types of light emission.

Note that all layers formed between the anode and the cathode aredefined as EL layers in this specification. Specifically, layers such asa light emitting layer, a hole injecting layer, an electron injectinglayer, a hole transporting layer, and an electron transporting layer areincluded as EL layers. An EL element basically has a structure in whichan anode, a light emitting layer, and a cathode are laminated in order.In addition to this structure, the EL element may also have a structurein which an anode, a hole injecting layer, a light emitting layer, and acathode are laminated in order, or a structure in which an anode, a holeinjecting layer, a light emitting layer, an electron transporting layer,and a cathode are laminated in order.

Furthermore, an element formed by an anode, an EL layer, and a cathodeis referred to as an EL element within this specification.

Circuit operation of an active matrix electronic device is explainednext with reference to FIGS. 13A and 13B. First, a voltage is applied tothe gate electrode of the switching TFT 1301 when the gate signal line1305 is selected, and the switching TFT 1301 is placed in a conductingstate. The signal (voltage) of the source signal line 1306 is thusstored in the storage capacitor 1304. The voltage of the storagecapacitor 1304 becomes a voltage V_(GS) between the gate and the sourceof the EL driver TFT 1302, and therefore the electric current inresponse to the storage capacitor 1304 voltage flows in the EL driverTFT 1302 and in the EL element 1303. As a result, the EL element 1303turns on.

The brightness of the EL element 1303, namely the amount of electriccurrent flowing in the EL element 1303, can be controlled by V_(GS) ofthe EL driver TFT 1302. V_(GS) is the voltage stored in the storagecapacitor 1304, and is the signal (voltage) inputted to the sourcesignal line 1306. In other words, the brightness of the EL element 1303is controlled by controlling the signal (voltage) of the source signalline 1306. Finally, the gate signal line 1305 is unselected, the gate ofthe switching TFT 1301 is closed, and the switching TFT 1301 is placedin a non-conducting state. The electric charge stored in the storagecapacitor 1304 continues to be stored at this point. V_(GS) of the ELdriver 1302 is therefore stored as is, and the electric current inresponse to V_(GS) continues to flow in the EL driver TFT 1302 and inthe EL element 1303.

Information regarding the above explanation is reported upon in paperssuch as the following: “Current Status and Future of Light-EmittingPolymer Display Driven by poly-Si TFT”, SID99 Digest, p. 372; “HighResolution Light Emitting Polymer Display Driven by Low TemperaturePolysilicon Thin Film Transistor with Integrated Driver”, ASIA DISPLAY98, p. 217; and “3.8 Green OLED with Low Temperature Poly-Si TFT”, EuroDisplay 99 Late News, p. 27.

Analog gray scale methods and digital gray scale methods exist asmethods of gray scale expression for an EL display. In the analog grayscale method, the value of V_(GS) of the EL driver TFT 1302 is changed,the amount of electric current flowing in the EL element 1303 iscontrolled, and the brightness is changed in an analog manner. In thedigital gray scale method, on the other hand, the voltage between thegate and the source of the EL driver TFT operates at only two levels: arange in which no electric current flows in the EL element 1303 (equalto or less than the turn on start voltage); and a range in which themaximum electric current flows (equal to or greater than the brightnesssaturation voltage). In other words, the EL element 1303 only takesturned-on and turned-off states.

EL displays mainly use the digital gray scale method, in whichdispersion in characteristics such as the threshold voltage of a TFTdoes not easily influence display. However, only two-gray-scale display,turned on and turned off, can be performed as is with the digital grayscale method, and therefore plural techniques capable of multiple grayscales by combining the digital gray scale method with another methodhave been proposed.

One of these techniques is a method in which a surface area gray scalemethod and a digital gray scale method are combined. The surface areagray scale method is a method of outputting gray scales by controllingthe surface area of portions which are switched on. Namely, one pixel isdivided into a plurality of sub-pixels, and the number of sub-pixelsturned on and the surface area are controlled, and a gray scale isexpressed.

FIGS. 14A and 14B are examples of pixel structures for performing grayscale expression in accordance with the surface area gray scale method.A region surrounded by a dotted line frame 1400 in FIG. 14A is a onepixel portion circuit. An enlarged diagram is shown in FIG. 14B.Reference numeral 1401 denotes a first switching TFT, reference numeral1402 denotes a second switching TFT, reference 1403 denotes a first ELdriver TFT, 1404 denotes a second EL driver TFT, 1405 denotes a first ELelement, 1406 denotes a second EL element, and reference numeral 1407 isa third EL element. Reference numeral 1408 denotes a first storagecapacitor, reference numeral 1409 denotes a second storage capacitor,1410 denotes a gate signal line, 1411 denotes a first source signalline, 1412 denotes a second source signal line, and reference numeral1413 is an electric current supply line.

The first switching TFT 1401 and the second switching TFT 1402 are firstplaced in a conducting state by selecting the gate signal line 1410 inthe gray scale expression method. When a signal is not inputted to thesource signal line, no EL elements turn on (gray scale 0). When a signalis inputted to the first source signal line 1411, the first EL driverTFT 1403 is placed into a conducting state via the first switching TFT1401, electric current is supplied to the first EL element 1405, and itturns on. A signal is not inputted to the second source signal line 1412at this point, and the second EL element 1406 and the third EL element1407 are in OFF states (gray scale 1). Next, if a signal is inputted tothe second source signal line 1412, then the second EL driver TFT 1404is placed in a conducting state via the second switching TFT 1402,electric current is supplied to the second EL element 1406 and the thirdEL element 1407, and they turn on. A signal is not inputted to the firstsource signal line 1411 at this point, and the first EL element 1405 isin a turned-off state (gray scale 2). Finally, when a signal is inputtedto both the first source signal line 1411 and the second source signalline 1412, the first EL driver TFT 1403 and the second EL driver TFT1404 are placed in conducting states via the first switching TFT 1401and the second switching TFT 1402, electric current is supplied to thefirst EL element 1405, the second EL element 1406, and the third ELelement 1407, and they turn on. All of the EL elements of one pixel turnon at this stage (gray scale 3). Four levels of gray scale expressioncan thus be performed in the pixel shown in FIGS. 14A and 14B.

Note that, in order to clarify the surface area of the turned on ELelements in FIGS. 14A and 14B, the second and the third EL elements areshown separately, but it is of course also possible to arrange only thesecond EL element having a surface area equal to twice that of the firstEL element.

Disadvantages of this method include fact that it is difficult toincrease the resolution, and fact that it is difficult to make a lot ofgray scales, because the number of sub-pixels cannot be made largewithout limits. The surface area gray scale method is reported in paperssuch as: “TFT-LEPD with Image Uniformity by Area Ratio Gray Scale”, EuroDisplay 99 Late News, p. 71; and “Technology for Active Matrix LightEmitting Polymer Displays”, IEDM 99, p. 107.

Another method capable of making many gray scales is a method whichcombines a time gray scale method and a digital gray scale method. Thetime gray scale method is a method of outputting gray scales bycontrolling the amount of turn on time. In other words, one frame periodis divided into a plurality of subframe periods, and gray scales areexpressed by controlling the number and length of the subframe periodsturned on.

A case of combining the digital gray scale method, the surface area grayscale method, and the time gray scale method is reported in“Low-Temperature Poly-Si TFT driven Light-Emitting-Polymer Displays andDigital Gray Scale for Uniformity”, IDW' 99, p. 171.

FIGS. 15A and 15B are timing charts for a driving method in whichdigital gray scales and time gray scales are combined. FIG. 15A shows atiming chart for a case in which address (write in) periods and sustain(turn on) periods are completely separated within a subframe period,while a case in which they are not separated is shown in FIG. 15B.

It is normally necessary to form address (write in) periods and sustain(turn on) periods corresponding to the number of bits in a drivingmethod utilizing time gray scales. With a driving method in which theaddress (write in) periods and sustain (turn on) periods are completelyseparated (a driving method in which the sustain (turn on) period beginsafter the address (write in) period of one pixel portion completelyfinishes in each subframe period), the proportion within one frameperiod occupied by the address (write in) period becomes large. Further,as shown in FIG. 15A, a period 1501 develops, in which write in and turnon cannot be performed in other rows, during a period in which the gatesignal line of a certain row is selected within the address (write in)period. The duty ratio (the length proportion of the sustain (turn on)period within one frame period) is thus greatly reduced. Increasing theoperational clock frequency is the only way to shorten the address(write in) period, and considering things such as the circuit operatingmargin, there are limits to making multiple gray scales. Conversely,with a driving method in which the address (write in) periods andsustain (turn on) periods are not separated, for example, the sustain(turn on) period for the EL element of a number k row, beginsimmediately after the selection period for the gate signal line of thenumber k row is completed. Therefore, the pixel is placed in an ON stateeven during times when the gate signal line is selected by other rows.This is consequently an advantageous driving method for making the dutyratio higher.

Problems such as the following appear, however, when the address (writein) periods and the sustain (turn on) periods are not separated. Thelength of one address (write in) period is from the start of theselection period for the first row gate signal line until the completionof the selection period of the last row gate signal line. At a certainpoint, selection of two differing gate signal lines cannot be performed,and therefore it is necessary for the sustain (turn on) period to have alength at least the same as, or greater than, the length of the address(write in) period for a driving method in which the address (write in)periods and sustain (turn on) periods are not separated. The ability tomake multiple gray scales is therefore limited by the minimum unit ofthe sustain (turn on) period. In FIG. 15B, the length of a portion shownby reference numeral 1502, in which a period up through the completionof an address (write in) period Ta₄ of the least significant bit portionof a subframe period SF₄ does not overlap with a period from thebeginning of the first address (write in) period of the next frameperiod, becomes the minimum unit. Normal display cannot be performed ifthe sustain (turn on) period has a shorter length. The length of theminimum unit of the sustain (turn on) period Ts_(min) is expressed byTs_(min)=Ta_(n)−Tg_(n) if the length of the address (write in) period istaken as Ta_(n) and the length of the selection period for one gatesignal line is taken as Tg_(n). The lengths of the sustain (turn on)periods for cases in which the digital gray scale method is combinedwith the time gray scale method are therefore determined by ratios ofpowers of 2, and considering the length of one frame period, it becomesdifficult to realize multiple gray scales.

A problem in which the minimum unit of the sustain (turn on) period islimited for cases in which the address (write in) period and the sustain(turn on) period are not separated is stated in the above timing charts.The following display method has been proposed in order to resolve thisproblem.

A sustain (turn on) period Ts₃ which is shorter than the minimum unitTs_(min) is contained within one frame period, and therefore a portionof Ta₃ and a portion of the next frame period Ta₁ which starts after thecompletion of Ts₃ are in a state of overlapping in a region denoted byreference numeral 1601 in FIG. 16A. Gate signal lines of differingcolumns are selected at the same time with this type of overlap portion,and therefore normal scanning cannot be performed. As shown in FIG. 16B,a period 1602 in which the EL element is in a non-display state is thenformed in the period in which, the address (write in) period overlapsafter the completion of the sustain (turn on) period having a lengthshorter than the minimum unit Ts_(min), and the start timing of the nextaddress (write in) period is delayed. Overlap of the address (write in)period disappears even when sustain (turn on) periods shorter than theminimum unit Ts_(min) are included, and normal display can consequentlybe performed.

FIGS. 17A and 17B show pixel structures recorded in Japanese PatentApplication No. Hei 11-338786 (applied on Nov. 29, 1999). A rangecontained within a dotted line frame 1700 in FIG. 17A is one pixelportion. FIG. 17B shows an enlargement diagram of FIG. 17A. Adding tothe structure of the pixel shown in FIGS. 13A and 13B, the structures ofFIGS. 17A and 17B have a structure in which a reset TFT 1705 and a resetsignal line 1712 are added.

Operation of the circuits shown in FIGS. 17A and 17B is explainedsimply. The operation relating to display of an image is similar to thatof a conventional pixel as shown in FIGS. 13A and 13B. The reset TFT1705 and the reset signal line 1712 are used when forming the abovestated non-display periods. The gate voltage applied to the EL driverTFT 1702 in the sustain (turn on) period (the electric potential of thegate electrode of the EL driver TFT 1702 with respect to the sourceregion) is provided in accordance with an electric charge stored by astorage capacitor 1704. Namely, the gate voltage applied to the ELdriver TFT 1702 (the electric potential of the gate electrode of the ELdriver TFT 1702 with respect to the source region) is equal to theelectric potential difference between both terminals of the storagecapacitor 1704. A reset signal is inputted to the reset signal line 1712to make the reset TFT 1705 in a conducting state in order to form thenon-display period after the completion of the sustain (turn on) period.The electric potential difference between the source region and thedrain region of the reset TFT 1705, namely the electric potentialdifference between both the terminals of the storage capacitor 1704,becomes 0 V by this operation. The voltage between the gate and thesource of the EL driver TFT 1702 therefore becomes 0 V and anon-conducting state is entered. Electric current supply to an ELelement 1703 is cut off. The reset TFT 1705 immediately returns to anon-conducting state, but the electric potential difference between boththe terminals of the storage capacitor 1704 is maintained as is at 0 V,and therefore the voltage between the gate and the source of the ELdriver TFT 1702 also remains at 0 V. The EL element 1703 does not turnon until a new image signal is next written in. The non-display periodhas at least a length which is found by the equation tr=ta−(ts+tg),where the length of the address (write in) period is taken as ta, thelength of the sustain (turn on) period is taken as ts, the length of onegate signal line selection period is taken as tg (with ta, ts, tg>0),and the length of the non-display period is taken as tr (where tr>0).The overlap of the address (write in) periods sandwiching a shortsustain (turn on) period can thus be avoided.

However, when using a pixel like that shown in FIGS. 17A and 17B,problems such as the following exist.

As stated above, it is preferable to use a p-channel TFT for the ELdriver TFT 1702. The threshold voltage is normally negative when using ap-channel TFT. Consequently, almost no drain current flows when thevoltage between the gate and the source of the EL driver TFT 1702 isequal to or greater than 0 V. However, a drain current flows in the ELdriver TFT 1702 when passing through the sustain (turn on) period, andtherefore this is a condition in which deterioration occurs easily whencompared to other TFTs. There are cases in which this varyingdeterioration and manufacturing irregularities become causes for a shiftof the threshold voltage to a positive number. In that case, the draincurrent flows even if the voltage between the gate and the source is 0V.

Consider, with reference to FIGS. 17A and 17B, a case in which thethreshold voltage of the EL driver TFT 1702 is actually shifted to apositive value. Further, description is given with respect to the periodin which a signal is normally written in. When a signal is inputted froma source signal line 1707 and black display (the EL element 1703 doesnot turn on) is performed, the voltage between the gate and the sourceof the EL driver TFT 1702 certainly becomes a positive value and thedrain current does not flow, provided that the electric potential of thesignal inputted from the source signal line 1707 is sufficiently higherthan the electric potential of an electric current supply line 1708. Inother words, by controlling the signal inputted form the outside, normaloperation becomes possible even for cases in which TFTs havingirregularities such as stated above are included.

On the other hand, with the operation in the non-display period in whichthe reset TFT 175 is made conductive and the electric current supply tothe EL element 1703 is cutoff, the electric potential of the sourcesignal line 1707 and the electric potential of the electric currentsupply line 1708 become equal in accordance with the reset TFT 1705. Thevoltage between the gate and the source of the EL driver TFT 1702becomes 0 V at this time, and a drain current flows if the thresholdvoltage is shifted to a positive value, and the EL element 1703 emitslight. This cannot be handled even if the electric potential of eachsignal line is changed.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a novelmethod of driving, in which a high duty ratio is maintained in anelectronic device performing driving as stated above, which is capableof normally performing display of an image even for cases having asustain (turn on) period shorter than the above stated minimum unit, andwhich is capable of handling cases in which problems such as the abovestated shift in the threshold value develop.

Further, when the term development of a shift in the threshold value ofthe TFT, or the term characteristic irregularity, are used throughoutthis specification, they indicate that the TFT characteristics arenormally on (the TFT is in a conducting state when the electricpotential difference between the gate electrode and the source region ofthe TFT is 0 V).

The following means are stated in the present invention in order tosolve the above problems.

One of a source region and a drain region of a reset TFT 105 iselectrically connected to an electric current supply line 108, and theother is electrically connected to a gate signal line 106. Further, aTFT having the same polarity as that of an EL driver TFT is used for aswitching TFT 101.

The voltage between a gate and a source of an EL driver TFT 102 in thestate that the reset TFT 105 is made conductive can be controlled bychanging the electric potential of the gate signal line 106 with thepresent invention. In accordance with this type of method, even if thethreshold voltage of the EL driver TFT 102 is shifted, becoming anormally ON state, the EL driver TFT 102 can be placed in anon-conductive state with certainty by changing the electric potentialof the gate signal line 106. It therefore becomes possible to make itdifficult for electric current to flow in an EL element 103.

Structures of an electronic device of the present invention are statedbelow.

According to a first aspect of the present invention, there is providedan electronic device comprising: a source signal line driver circuit; agate signal line driver circuit; a reset signal line driver circuit; anda pixel portion;

characterized in that:

-   -   the pixel portion has: a plurality of source signal lines; a        plurality of gate signal lines; a plurality of electric current        supply lines; a plurality of reset lines; and a plurality of        pixels;    -   each of the plurality of pixels has: a switching transistor; an        EL driver transistor; a reset transistor; a storage capacitor,        and an EL element;    -   a gate electrode of the switching transistor is electrically        connected to one of the plurality of gate signal lines;    -   one of a source region and a drain region of the switching        transistor is electrically connected to the source signal line,        and the remaining one of the source region and the drain region        is electrically connected to a gate electrode of the EL driver        transistor;    -   a gate electrode of the reset transistor is electrically        connected to the reset signal line;    -   one of a source region and a drain region of the reset        transistor is electrically connected to one of the plurality of        gate signal lines, and the remaining one of the source region        and the drain region is electrically connected to a gate        electrode of the EL driver transistor;    -   one electrode of the storage capacitor is electrically connected        to the electric current supply line, and the remaining electrode        is electrically connected to the gate electrode of the EL driver        transistor; and    -   one of a source region and a drain region of the EL driver        transistor is electrically connected to the electric current        supply line, and the remaining one of the source region and the        drain region is electrically connected to one electrode of the        EL element.

According to a second aspect of the present invention, in the firstaspect of the present invention, there is provided the electronicdevice, characterized in that:

-   -   a p-channel polarity transistor is used for the switching        transistor when the source region or the drain region of the EL        driver transistor is electrically connected to an anode of the        EL element; and    -   an n-channel polarity transistor is used for the switching        transistor when the source region or the drain region of the EL        driver transistor is electrically connected to a cathode of the        EL element.

According to a third aspect of the present invention, there is provideda method of driving an electronic device, characterized in that:

-   -   one frame period has n subframe periods SF₁, SF₂, . . . ,        SF_(n);    -   the n subframe periods each have address (write in) periods Ta₁,        Ta₂, . . . , Ta_(n), and sustain (turn on) periods Ts₁, Ts₂, . .        . , Ts_(n);    -   the address (write in) period and the sustain (turn on) period        overlap in at least one subframe period among the n subframe        periods; and    -   when the address (write in) period Ta_(m) (where 1≦m≦n) of the        subframe period SF_(m), and the address (write in) period        Ta_(m+1), of the subframe period SF_(m+1) overlap, a non-display        period exists in a period from the completion of the sustain        (turn on) period SF_(m) of the subframe period SF_(m), until the        start of the address (write in) period Ta_(m+1).

According to a fourth aspect of the present invention, there is provideda method of driving an electronic device, characterized in that:

-   -   one frame period has n subframe periods SF₁, SF₂, . . . ,        SF_(n);    -   the n subframe periods each have address (write in) periods Ta₁,        Ta₂, . . . , Ta_(n), and sustain (turn on) periods Ts₁, Ts₂, . .        . , Ts_(n);    -   the address (write in) period and the sustain (turn on) period        overlap in at least one subframe period among the n subframe        periods; and    -   when the address (write in) period Ta_(n) of the subframe period        SF_(n) of the number j frame (where 0<j), and the address (write        in) period Ta₁ of the subframe period SF₁ of the number j+1        frame overlap, a non-display period exists in a period from the        completion of the sustain (turn on) period Ts_(n) of the        subframe period SF_(n) of the number j frame, until the start of        the address (write in) period Ta₁ of the subframe period SF of        the number j+1 frame.

According to a fifth aspect of the present invention, there is provideda method of driving an electronic device, characterized in that:

-   -   one frame period has n subframe periods SF₁, SF₂, . . . ,        SF_(n);    -   the n subframe periods each have address (write in) periods Ta₁,        Ta₂, . . . , Ta_(n), and sustain (turn on) periods Ts₁, Ts₂, . .        . , Ts_(n); and    -   if ta_(k)>ts_(k)+tg is satisfied in a certain subframe period        SF_(k) (1≦k≦n), where the length of the address (write in)        period is taken as ta_(k), the length of the sustain (turn on)        period is taken as ts_(k) and the length of one gate signal line        selection period is taken as tg, and ta_(k), ts_(k), tg>0, and        if the length of a non-display period of SF_(k) is taken as        tr_(k) (where tr_(k)>0):

tr _(k) ≧ta _(k)−(ts _(k) +tg) is always satisfied.

According to a sixth aspect of the present invention, in any of thethird to fifth aspects of the present invention, there is provided themethod of driving an electronic device, characterized in that:

-   -   the EL driver transistor is placed in a non-conductive state in        the non-display period by the reset transistor being conductive        due to a signal input from a reset signal line driver circuit;        and    -   during a period after the reset transistor returns to a        non-conducting state, until write in of the next signal from the        source signal line is preformed, the gate voltage of the EL        driver transistor is maintained by the storage capacitor.

According to a seventh aspect of the present invention, in any of thethird to sixth aspects of the present invention, there is provided themethod of driving an electronic device, characterized in that the ELelement is turned off during the non-display period, irrespective of animage signal.

According to an eighth aspect of the present invention, in any of thethird to seventh aspects of the present invention, there is provided themethod of driving an electronic device, characterized in that the gatevoltage of the EL driver transistor in the non-display period isdetermined by the difference between the electric potential of theelectric current supply line and the electric potential of a certaingate signal lire in a non-selected state.

According to a ninth aspect of the present invention, in any of thethird to eighth aspects of the present invention, there is provided themethod of driving an electronic device, characterized in that anelectric potential lower than the threshold voltage of the EL drivertransistor, with respect to the electric potential of the electriccurrent supply line, is inputted to the gate signal line in anon-selected state for a case in which the EL driver transistor hasn-channel polarity.

According to a tenth aspect of the present invention, in any of thethird to eighth aspects of the present invention, there is provided themethod of driving an electronic device, characterized in that anelectric potential higher than the threshold voltage of the EL drivertransistor, with respect to the electric potential of the electriccurrent supply line, is inputted to the gate signal line in anon-selected state for a case in which the EL driver transistor hasp-channel polarity.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams showing circuit structures of an electronicdevice of the present invention;

FIGS. 2A to 2C are diagrams showing the relationship among electricpotentials of respective portions in a pixel portion;

FIGS. 3A and 3B are diagrams showing examples of circuit structuresusing the pixel of the present invention according to Embodiment 1;

FIGS. 4A and 4B are diagrams showing timing charts relating to a methodof driving according to Embodiment 1;

FIG. 5 is a diagram showing a timing chart of a gate signal line and areset signal line in the method of driving according to Embodiment 1;

FIGS. 6A to 6C are diagrams showing an example of a process ofmanufacturing an electronic device according to Embodiment 2;

FIGS. 7A to 7C are diagrams showing an example of the process ofmanufacturing the electronic device according to Embodiment 2;

FIGS. 8A and 8B are diagrams showing an example of the process ofmanufacturing the electronic device according to Embodiment 2;

FIGS. 9A and 9B are a top surface diagram and a cross sectional diagram,respectively, of an electronic device according to Embodiment 3;

FIG. 10 is a cross sectional diagram of a pixel portion of an electronicdevice according to Embodiment 4;

FIG. 11 is a cross sectional diagram of a pixel portion of an electronicdevice according to Embodiment 5;

FIG. 12 is a cross sectional diagram of a pixel portion of an electronicdevice according to Embodiment 6;

FIGS. 13A and 13B are diagrams showing examples of electronic devicecircuit structures;

FIGS. 14A and 14B are diagrams showing examples of a pixel portion of anelectronic device for performing gray scale expression in accordancewith a surface area gray scale method;

FIGS. 15A and 15B are diagrams showing timing charts for explainingframe period division in time gray scaling;

FIGS. 16A and 16B are diagrams showing overlap of an address (write in)period, and a method of solving in accordance with a non-display period,respectively;

FIGS. 17A and 17B are diagrams showing the structure of the pixelrecorded in Japanese Patent Application No. Hei 11-338786;

FIGS. 18A and 18B are diagrams showing examples of the circuitstructures according to Embodiment 7 using the pixel of the presentinvention;

FIGS. 19A and 19B are diagrams showing examples of the circuitstructures according to Embodiment 8 using the pixel of the presentinvention;

FIGS. 20A to 20F are diagrams showing examples of electronic equipment,according to Embodiment 11, which apply the method of driving anelectronic device of the present invention; and

FIGS. 21A and 21B are diagrams showing examples of electronic equipment,according to Embodiment 11, which apply the method of driving anelectronic device of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment mode of the present invention is discussed below.

The pixel recorded in Japanese Patent Application No. Hie 11-338786 isone in which one of the source region and the drain region of the resetTFT 1705 is electrically connected to the electric current supply line1708, and the other is electrically connected to a gate electrode of theEL driver TFT 1702, as shown in FIGS. 17A and 17B. The gate electrode ofthe reset TFT 1705 is electrically connected to the reset signal line1712.

A pixel of the present invention is one in which one of the sourceregion and the drain region of a reset TFT 105 is electrically connectedto an electric current supply line 108, while the other is electricallyconnected to a gate signal line 106, as shown in FIGS. 1A and 1B.

Electric potential patterns in each wiring are explained next, withreference to FIGS. 2A to 2C. FIG. 2A shows the electric potential of areset signal line, and FIG. 2B shows the electric potential of eachwiring for a case of performing drive involving non-display periodsusing the pixel recorded in Japanese Patent Application No. Hei11-338786 and shown in FIGS. 17A and 17B. FIG. 2C shows the electricpotential of each wiring for a case of performing drive involvingnon-display periods using a pixel having the structure of the presentinvention. The case of FIG. 2B is explained first. Note that in order toclearly show the electric potential of each portion, the explanation ismade with an n-channel TFT used for the switching TFT and with p-channelTFTs used for the EL driver TFT and for the reset TFT.

A signal waveform 201 shown in FIG. 2A is for a case of using ap-channel TFT for the reset TFT 1705, and the reset TFT 1705 is placedin a conducting state when the electric potential drops. The waveform201 of FIG. 2A becomes the inverse if an n-channel TFT is used in thereset TFT 1705.

An electric potential 202 of the gate signal line 1706 is discussednext. An n-channel TFT is used for the switching TFT 1701 in the case ofFIG. 2B. The electric potential therefore increases when the gate signalline 1706 is selected, and the switching TFT 1701 is placed in aconducting state.

An electric potential 204 of the source signal line 1707 is inputted tothe EL driver TFT 1702 and the storage capacitor 1704 via the switchingTFT 1701.

An electric potential 203 of the gate electrode of the EL driver TFT1702 becomes equal to the electric potential 204 of the source signalline 1707 when the switching TFT 1701 is in a conducting state. At thepoint where the switching TFT 1701 is placed in a conducting state inFIGS. 2A to 2C, the electric potential 204 of the source signal line1707 is a LO signal, and therefore the electric potential 203 of thegate electrode of the EL driver TFT 1702 drops. The absolute value ofthe voltage between the gate and the source of the EL driver TFT 1702becomes larger at this point, and the EL driver TFT 1702 is placed in aconducting state. Electric current therefore flows in the EL element1703, and the EL element 1703 turns on. If the electric potential 204 ofthe source signal line 1707 is HI, then the EL element 1703 does notturn on.

A LO signal is then inputted to the reset signal line 1712 at a timinagshown by the dashed line X-X′ in FIGS. 2A to 2C, and the reset TFT 1705is placed in a conducting state. The electric potential 203 of the gateelectrode of the EL driver TFT 1702 becomes equal to an electricpotential 205 of the electric current supply line 1708 by thisoperation, and the gate voltage of the EL driver TFT 1702 (the electricpotential of the gate electrode with respect to the source region of theEL driver TFT 1702) becomes 0 V. In other words, when the thresholdvoltage of the EL driver TFT 1702 is shifted to a positive value, it isplaced in a conducting state at the point when the gate voltage of theEL driver TFT 1702 (the electric potential of the gate electrode withrespect to the source region of the EL driver TFT 1702) becomes 0 V, andelectric current flows in the EL element 1703 also during thenon-display period. The non-display period cannot be formed normallyhere.

The case of FIG. 2C is explained next. In this case, the electricpotentials of respective portions are explained assuming that p-channelTFTs are used for the switching TFT, the EL driver TFT, and the resetTFT.

First, an electric potential 206 of the gate signal line 106 isexplained. As discussed above, a p-channel TFT is used for the switchingTFT 101, and therefore the electric potential drops when the gate signalline 106 is selected, and the switching TFT 101 is placed in aconducting state.

An electric potential 208 of the source signal line 107 is inputted tothe EL driver TFT 102 and to the storage capacitor 104 via the switchingTFT 101.

An electric potential 207 of the gate electrode of the EL driver TFT 103becomes equal to the electric potential 208 of the source signal line107 when the switching TFT 101 is in a conducting state. In FIGS. 2A to2C, the electric potential 208 of the source signal line 107 is a LOsignal at the point where the switching TFT 101 is placed in aconducting state, and therefore the electric potential 207 of the gateelectrode of the EL driver TFT 102 drops. The absolute value of thevoltage between the gate and the source of the EL driver TFT 102 becomeslarger at this point, and the EL driver TFT 102 is placed in aconducting state. Electric current consequently flows in the EL element103, and the EL element 103 turns on. The EL element 103 does not turnon for a case in which the electric potential 208 of the source signalline 107 is a HI signal.

A LO signal is then inputted to the reset signal line 112 at a timingshown by the dashed line X-X′ in FIGS. 2A to 2C, and the reset TFT 105is placed in a conducting state. The electric potential 207 of the gateelectrode of the EL driver TFT 102 becomes equal to the electricpotential 206 of the gate signal line 106 at this point. For a case inwhich the EL driver TFT is normally on, the voltage between the gate andthe source may be set to a positive value (when a p-channel TFT isused), and may be set so as to turn off with certainty. The voltagebetween the gate and the source of the EL driver TFT 102 can thereforetake on a positive value by making the electric potential 206 of thegate signal line 106 higher corresponding to the amount of shift in thethreshold value of the EL driver TFT 102. Electric current can thereforebe prevented from flowing even if the threshold value of the EL driverTFT 102 shifts to a positive value, differing from the case of FIG. 2B.

After the reset TFT 105 returns to a non-conducting state, the voltagebetween the gate and the source of the EL driver TFT 102 at that pointis stored by the storage capacitor 104, and therefore the EL element 103continues in an OFF state in the next subframe period until write in ofa signal to the pixel is performed.

The relationship between the polarity of the TFTs structuring the pixelsand the electric potential of each portion is explained next.

1) Using an n-Channel TFT for the EL Driver TFT:

It is necessary to make the voltage V_(GS) between the gate and thesource of the EL driver TFT 102 lower than the threshold voltage so thatthe EL driver TFT 102 will be placed in a non-conducting state withcertainty in the non-display period. The gate electric potential of theEL driver TFT 102 becomes an electric potential V_(G) of the gate signalline 106, and the source electric potential becomes an electricpotential V_(CUL) of the electric current supply line 108 at this pointbecause the reset TFT 105 is made conductive. If the EL driver TFT 102is normally on, then at least V_(G)<V_(CUL) must be true. The electricpotential V_(G) of the gate signal line 106 arbitrarily changes alongwith deterioration of the EL driver TFT 102, but in this case V_(G)changes in the direction of becoming lower as deterioration proceeds.Therefore, in order to place the switching TFT 101 in a non-conductingstate for all cases, even if the gate electric potential of theswitching TFT 101, namely the electric potential V_(G) of the gatesignal line 106, takes a low value, the switching TFT 101 must always bein a non-conducting state. It is thus preferable to use an n-channel TFTfor the switching TFT 101.

2) Using a p-Channel TFT for the EL Driver TFT:

It is necessary to make the voltage V_(GS) between the gate and thesource of the EL driver TFT 102 higher than the threshold voltage sothat the EL driver TFT 102 will be placed in a non-conducting state withcertainty in the non-display period. The gate electric potential of theEL driver TFT 102 becomes the electric potential V_(G) of the gatesignal line 106, and the source electric potential becomes the electricpotential V_(CUL) of the electric current supply line 108 at this pointbecause the reset TFT 105 is made conductive. If the EL driver TFT 102is normally on, then at least V_(G)>V_(CUL) must be true. The electricpotential V_(G) of the gate signal line 106 arbitrarily changes alongwith deterioration of the EL driver TFT 102, but in this case V_(G)changes in the direction of becoming higher as deterioration proceeds.Therefore, in order to place the switching TFT 101 in a non-conductingstate for all cases, even if the gate electric potential of theswitching TFT 101, namely the electric potential V_(G) of the gatesignal line 106, takes a high value, the switching TFT 101 must alwaysbe in a non-conducting state. It is thus preferable to use a p-channelTFT for the switching TFT 101.

Note that the polarity of the reset TFT 105 is not particularly ofconcern, but considering the voltage between the source and the drain ofthe reset TFT 105, it is preferable to use an n-channel TFT for thefirst case above, and it is preferable to use a p-channel TFT for thesecond case above.

Note also that, although one of the source region and the drain regionof the reset TT 105, and the gate electrode of the switching TFT 101,are both electrically connected to the same gate signal line 106 inFIGS. 1A and 1B, one of the source region and the drain region of thereset TFT 105, may also be connected to any gate signal line, not onlythe gate signal line 106 within FIGS. 1A and 1B.

Further, although a case of a driving method which combines a time grayscale method and a digital gray scale method is discussed in theembodiment mode, the essence of the present invention, the arrangementof the reset TFT, can also be applied to other methods of driving. Itcan also be applied, of course, to a driving method in which a surfacearea gray scale method and a digital gray scale method are combined, andto a method of driving in which a surface area gray scale method, adigital gray scale method, and a time gray scale method are combined.

Embodiments of the present invention are described below.

Embodiment 1

FIG. 3A is an example of an entire circuit structure of an electronicdevice shown by Embodiment 1. A pixel portion 351 is arranged in thecenter of a substrate 350. A source signal line driver circuit 352 isarranged on the top side of the pixel portion 351 in order to controlsource signal lines. To the left side of the pixel portion 351 isarranged a gate signal line driver circuit 353 in order to control gatesignal lines. A reset signal line driver circuit 354 is arranged on theright side of the pixel portion 351 in order to control reset signallines. A portion surrounded by a dotted line frame 300 in the pixelportion 351 is one pixel portion circuit, and an enlargement diagram isshown in FIG. 3B. The names of respective portions of FIG. 3B aresimilar to those of FIG. 1B, and therefore are omitted here.

Actual driving is discussed next. In Embodiment 1, k-bit (2^(k)) grayscales are expressed by a method in which digital gray scales and timegray scales are combined. For simplicity of explanation, an example of acase in which 3-bit gray scale expression is performed, with k=3, isexplained. The circuit shown in FIGS. 3A and 3B is referenced.

Timing charts for the 3-bit gray scale expression explained inEmbodiment 1 are shown in FIGS. 4A and 4B. One frame period is dividedinto three subframe periods SF₁ to SF₃, and the subframe periods haveaddress (write in) periods Ta₁ to Ta₃ and sustain (turn on) periods Ts₁to Ts₃, respectively. The lengths of the sustain (turn on) periods areset so as to be powers of 2, and for FIGS. 4A and 4B, this becomesTs₁:Ts₂:Ts₃=2²:2¹:2⁰.

Further, the address (write in) periods are periods from the selectionof the first row gate signal line until the completion of selection ofthe last row gate signal line, and therefore Ta₁ to Ta₃ all have equallengths.

The sustain (torn on) period Ts₃ of the least significant bit portion isshorter than the address (write in) period Ta₃. Therefore, as shown inFIG. 4A, if there is a transition to the address (write in) period Ta₁of the next frame period immediately after the completion of the sustain(turn on) period Ts₃, a period develops in which the address (write in)periods of differing subframe periods overlap. Selection of a pluralityof gate signal lines is performed simultaneously in this period, andtherefore normal image display cannot be performed.

A signal is inputted to a reset signal line 312 after the completion ofthe sustain (turn on) period Ta₃, as shown in FIG. 4B, an EL element 303is turned off, and a non-display period is formed in a period until thestart of the next address (write in) period. The electric potential of agate signal line 306 and the reset signal line 312 in a certain frameperiod are shown in FIG. 5. A p-channel TFT is used as a reset TFT 305in Embodiment 1, and therefore the reset TFT 305 is placed in aconducting state when the electric potential of the reset signal line312 is low. An n-channel TFT may also be used as the reset TFT 305.

First, the gate signal line 306 is selected in the subframe period SF₁,and write in of a signal to a pixel from a source signal line 307 isperformed. When signal write in to pixels is completed for each row, thesustain (turn on) period Ts₁ immediately begins. This operation isperformed from the first row until the last row. The gate signal line306 is then similarly selected in the subframe period SF₂, and write inof a signal to a pixel from the source signal line 307 is performed. Thesustain (turn on) period Ts₂ immediately begins when signal write in tothe pixels is completed for each row. This operation is performed fromthe first row until the last row.

In the subframe period SF₃, first the gate signal line 306 is selected,similar to SF₁ and SF₂, and write in of a signal to a pixel isperformed. After signal write in to the pixel is completed for each row,the sustain (turn on) period Ts₃ immediately begins. This operation isperformed from the first row until the last row. The sustain (turn on)period Ts₃ is shorter than the address (write in) period Ta₃ at thispoint, and consequently, before the completion of the address (write in)period Ta₃, namely before the completion of the period for selecting thegate signal line of the final row, the sustain (turn on) period Ts₃ ofthe first row is completed. Immediately after the sustain (turn on)period Ts₃ of the first row is completed, a reset signal is inputted tothe reset signal line of the first row, the reset TFT 305 is placed in aconducting state, and the electric potential difference between bothelectrodes of a storage capacitor 304, namely the voltage between thegate and the source of the EL driver TFT 302, becomes equal to theelectric potential difference between the gate signal line 306 and anelectric current supply line 308. The EL driver TFT 302 therefore isplaced in a non-conducting state, and the supply of electric current tothe EL element 303 is cutoff. The voltage between the gate and thesource of the EL driver TFT 302 at this point is stored by the storagecapacitor 304 even after the reset TFT 305 returns to a non-conductingstate. The EL element 303 therefore continues in a turned OFF state inthe next subframe period until write in of a signal to the pixel isperformed.

The electric potential of the gate signal line 306 in a non-selectedstate need to be raised in a case where the threshold voltage of the ELdriver TFT 302 is shifted to a positive value. By doing so, the electricpotential difference between both electrodes of the storage capacitor304, namely the gate voltage of the EL driver TFT 302 (the electricpotential of the gate electrode with respect to the source region of theEL driver TFT 302) can be arbitrarily controlled.

In accordance with the method of driving shown in Embodiment 1, it ispossible to freely set the length of the sustain (turn on) period bychanging the timing at which the reset signal is input. It is possibleto perform normal display of an image even in a subframe period having asustain (turn on) period shorter than the minimum unit in the displaymethod which combines normal digital gray scales and time gray scales.

Further, even if the EL driver TFT 302 is normally on, it is possible tohandle this by changing the electric potential of the gate signal line306 in a non-selected state.

Embodiment 2

In Embodiment 2, a detailed description will be made of a method ofsimultaneously manufacturing, on the same substrate, a pixel portion anda driver circuit TFT (N-channel type and P-channel type) formed in theperiphery of the pixel portion is explained.

First, as shown in FIG. 6A, a base film 5002 made of an insulating filmsuch as a silicon oxide film, a silicon nitride film or a siliconoxynitride film is formed on a substrate 5001 made from glass such as abarium borosilicate glass or an alumino borosilicate glass, typically#7059 glass or #1737 glass of Corning Corp. For example, a siliconoxynitride film 5002 a formed from SiH₄, NH₃, N₂O by a plasma CVD methodwith a thickness of 10 to 200 nm (preferably 50 to 100 nm), and asilicon oxynitride hydride film 5002 b formed from SiH₄ and N₂O,similarly, with a thickness of 50 to 200 m (preferably 100 to 150 nm)are laminated. In Embodiment 2, the base film 5002 is shown as atwo-layer structure, but a single layer or a lamination of two or morelayers of the insulating film may be adopted to form the base film 5002.

Island-shape semiconductor layers 5003 to 5006 are formed of acrystalline semiconductor film obtained by crystallizing a semiconductorfilm having an amorphous structure with a laser crystallization methodor a thermal crystallization method. The island-shape semiconductorlayers 5003 to 5006 are given a thickness of from 25 to 80 nm(preferably 30 to 60 nm). There is no limitation on the material of thecrystalline semiconductor film, but silicon or an alloy of silicongermanium (SiGe) is preferably used. To form the crystallinesemiconductor film by the laser crystallization method, a pulseoscillation type or continuous emission type excimer laser, a YAG laser,or YVO₄ laser is used. In the case where the above laser is used, it isappropriate to use a method in which laser light radiated from a laseroscillator is condensed by an optical system into a linear beam, and isirradiated to a semiconductor film. Although the condition ofcrystallization should be appropriately selected by an operator, in thecase where the excimer laser is used, a pulse oscillation frequency isset to 30 Hz, and a laser energy density is set to 100 to 400 ml/cm²(typically 200 to 300 mJ/cm²). Also, in the case where the YAG laser isused, it is appropriate that the second harmonic is used, a pulseoscillation frequency is set to 1 to 10 kHz, and a laser energy densityis set to 300 to 600 mJ/cm² (typically, 350 to 500 mJ/cm²). Then, laserlight condensed into a linear shape with a width of 100 to 1000 μm, forexample, 400 μm is irradiated to the whole surface of the substrate, andan overlapping ratio (overlap ratio) of the linear laser light at thistime is set to 80 to 98%.

Next, a gate insulating film 5007 is formed, which covers theisland-shape semiconductor layers 5003 to 5006. The gate insulating film5007 is formed of an insulating film containing silicon with a thicknessof from 40 to 150 nm by using plasma CVD or sputtering. In Embodiment Zthe gate insulating film 5007 is formed of an oxynitride silicon filmwith a thickness of 120 nm. Of course, the gate insulating film is notlimited to such an oxynitride silicon film, and another insulating filmcontaining silicon may be used as a single-layer structure or alamination structure. For example, in the case where a silicon oxidefilm is used, TEOS (tetraethyl orthosilicate) and O₂ are mixed with eachother by the plasma CVD method, with reaction pressure of 40 Pa, asubstrate temperature of 300 to 400° C., and discharge is made at a highfrequency (13.56 MHz) with a power density of 0.5 to 0.8 W/cm², to formthe film. Then, the silicon oxide film thus formed is subjected tothermal annealing at 400 to 500° C. to obtain excellent characteristicsas the gate insulating film.

Then, a first conductive film 5008 and a second conductive film 5009,for forming a gate electrode on the gate insulating film 5007, areformed. In Embodiment 2, the first conductive film 5008 is formed of aTa film with a thickness of 50 to 100 nm, and the second conductive film5009 is formed of a W film with a thickness of 100 to 300 nm.

The Ta film is formed by sputtering with Ar using Ta as a target. Inthis case, when a suitable amount of Xe or Kr is added to Ar forsputtering, it is possible to relieve internal stress of the Ta film,thereby being capable of preventing the film from peeling off. Theresistivity of an α-phase Ta film is on the order of 20 μΩcm and thefilm can be used as the gate electrode. However, the resistivity of aβ-phase Ta film is on the order of 180 μΩm, and the film is not suitablefor the gate electrode. To form the α-phase Ta film, if tantalum nitridehaving crystalline structure close to the α-phase of Ta is formed to athickness of on the order of 10 to 50 nm as a base of Ta, the α-phase Tafilm can be easily obtained.

The W film is formed by sputtering with a W target. In addition, the Wfilm can be formed by thermal CVD using tungsten hexafluoride (WF₆).Whichever method is used, it is necessary to make the material have lowresistance for use as the gate electrode. It is preferable that theresistivity of the W film is set as 20 μΩcm or lower. By making thecrystal grains large, it is possible to make the W film have lowerresistivity. However, for cases where there are many impurity elementssuch as oxygen within the W film, crystallization is inhibited and theresistance becomes higher. Therefore, by forming the W film using a Wtarget having a purity of 99.9999%, and further, taking sufficientconsideration so that there is no mixing in of impurities from theinside of the gas phase during film formation, resistivity of 9 to 20μΩcm can be realized.

Note that, in Embodiment 2, although the Ta film and the W film are usedfor the first conductive film 5008 and the second conductive film 5009,respectively, there is no limitation on the material for the conductivefilms. The first conductive film 5008 and the second conductive film5009 may be formed from an element selected from the group consisting ofTa, W, Ti, Mo, Al, and Cu, or an alloy material or compound materialcontaining as a main component the above element. Also, a semiconductorfilm typified with a polycrystalline silicon film into which an impuritysuch as phosphorous is doped may be used. Examples of the othercombinations preferably include: a combination in which the firstconductive film is formed from tantalum nitride (TaN) and the secondconductive film is formed from W; a combination in which the firstconductive film is formed from tantalum nitride (TaN) and the secondconductive film is formed from Al; and a combination in which the firstconductive film is formed from tantalum nitride (TaN) and the secondconductive film is formed from Cu.

Then, a mask 5010 is formed from resist, and a first etching process forforming electrodes and wirings is performed. In Embodiment 2, an ICP(inductively coupled plasma) etching method is used, and the etching isperformed using a mixture of CF₄ and Cl₂ as etching gasses, and a 500 WRF (13.56 MHz) power is inputted to a coil type electrode at a pressureof 1 Pa to generate plasma. A 100 W RF (13.56 MHz) power is inputted tothe substrate side (sample stage) as well, substantially applying anegative self bias voltage. When CF₄ and Cl₂ are mixed together, the Wfilm and the Ta film can be etched at approximately the same rate.

Under the above-mentioned conditions, the shape of the mask consistingof resist is made into an appropriate one, with the result that edgeportions of the first conductive film and the second conductive film aremade into a tapered shape by the effect of a biasing voltage applied tothe substrate side. The angle of the tapered portion is set from 15 to45′. In order to etch without any residue remaining on the gateinsulating film, the etching time is preferably increased by a ratio onthe order of 10 to 20%. The selectivity of the oxynitride silicon filmis 2 to 4 (typically 3) with respect to the W film, and therefore theoxynitride silicon film is etched on the exposed surface byapproximately 20 to 50 nm due to an over-etching process. First shapeconductive layers 5011 to 5016 (first conductive layers 5011 a to 5016 aand second conductive layers 5011 b to 5016 b) are thus formed from thefirst conductive film and the second conductive film by the firstetching process. At this time, in the gate insulating film 5007, regionswhich are not covered with the first shape conductive layers 5011 to5016 are etched by about 20 to 50 nm so that thinned regions are formed(FIG. 6A).

Then, a first doping treatment is carried out to add an impurity elementimparting an n-type (FIG. 6B). Doping may be carried out by ion dopingor ion injecting. The condition of the ion doping method is such that adosage is 1×10¹³ to 5×10¹⁴ atoms/cm², and an acceleration voltage is 60to 100 keV. As the impurity element imparting the n-type, an elementbelonging to group 15, typically phosphorus (P) or arsenic (As) may beused. Phosphorus (P) is used here. In this case, the conductive layers5011 to 5015 become the masks to the impurity element imparting then-type, and first impurity regions 5017 to 5025 are formed in a selfaligning manner. An impurity element imparting the n-type in theconcentration range of 1×10²⁰ to 1×10²¹ atoms/cm³ is added to the firstimpurity regions 5017 to 5025.

A second etching process is performed next, as shown in FIG. 6C. The ICPetching method is similarly used, and a mixture of CF₄, Cl₂, and O₂ areused as an etching gas. An RF power (13.56 MHz) of 500 W is applied to acoil type electrode under a pressure of 1 Pa to generate plasma. An RF(13.56 MHz) power of 50 W is applied to the side of the substrate(sample stage) and a low self bias voltage as compared with the firstetching treatment is applied. The W film is anisotropically etched inaccordance with these conditions, and the Ta film is anisotropicallyetched at a slower etching rate, forming second shape conductive films5026 to 5031 (first conductive layers 5026 a to 5031 a and secondconductive layers 5026 b to 5031 b). Region not covered by the secondshape conductive layers 5026 to 5031 are further etched on the order of20 to 50 nm, so that thinned regions are formed.

An etching reaction of the W film or the Ta film by the mixture gas ofCF₄ and C₂ can be guessed from a generated radical or ion species andthe vapor pressure of a reaction product. When the vapor pressures offluoride and chloride of W and Ta are compared with each other, WF₆ offluoride of W is extremely high, and other WCl₅, TaF₅, and TaCl₅ havealmost equal vapor pressures. Thus, in the mixture gas of CF₄ and Cl₂,both the W film and the Ta film are etched. However, when a suitableamount of O₂ is added to this mixture gas, CF₄ and O₂ react with eachother to form CO and F, and a large number of F radicals or F ions aregenerated. As a result, an etching rate of the W film having the highvapor pressure of fluoride is increased. On the other hand, with respectto Ta, even if F is increased, an increase of the etching rate isrelatively small. Besides, since Ta is easily oxidized as compared withW, the surface of Ta is oxidized by an addition of O₂. Since the oxideof Ta does not react with fluorine or chlorine, the etching rate of theTa film is further decreased. Accordingly, it becomes possible to make adifference between the etching rates of the W film and the Ta film, withthe result that it becomes possible to make the etching rate of the Wfilm higher than that of the Ta film.

Then, as shown in FIG. 7A, a second doping treatment is carried out. Inthis case, a dosage is made lower than that of the first dopingtreatment and under the condition of a high acceleration voltage, and animpurity element imparting the n-type is doped. For example, anacceleration voltage is made 70 to 120 keV, and the treatment is carriedout at a dosage of 1×10¹³ atoms/cm², so that new impurity regions areformed inside the first impurity regions formed into the island-shapesemiconductor layers in FIG. 6B. Doping is carried out using as themasks the second shape conductive layers 5026 to 5030 to add theimpurity element to the regions under the second conductive layers 5026a and 5030 a. In this way, third impurity regions 5032 to 5041overlapping with the second conductive layers 5026 a and 5030 a, andsecond impurity regions 5042 to 5051 between the first impurity regionsand the third impurity regions are formed. The concentration of theimpurity element imparting the n-type is set so that the concentrationof the second impurity regions become 1×10¹⁷ to 1×10¹⁹ atoms/cm³, andthe concentration of the third impurity regions become 1×10¹⁶ to 1×10¹⁵atoms/cm³.

Then, as shown in FIG. 7B, fourth impurity regions 5052 to 5074 areformed, which have a reverse conductivity to that of the firstconductivity in an island-like semiconductor layer 5004 to 5006 forforming p-channel TFTs. Second conductive layers 5027 b to 5030 b areused as masks against impurity element to form the impurity regions in aself aligning manner. At this time, the whole surfaces of theisland-like semiconductor layer 5003 forming n-channel TFTs and a wiringportion 5031 are covered with resist masks 5200. Phosphorus is added tothe impurity regions 5522 to 5524 at different concentrations,respectively. However, the regions are formed by ion doping usingdiborane (B₂H₆) and the impurity concentration is made 2×10²⁰ to 2×10²¹atoms/cm³ in any of the regions.

By the steps up to this, the impurity regions are formed in therespective island-like semiconductor layers. The second conductivelayers 5026 to 5030 overlapping with the island-like semiconductor layerfunction as gate electrodes. Also, the wiring portion 5031 functions asan island-like source signal line.

A step of activating the impurity elements added in the respectiveisland-like semiconductor layers for the purpose of controlling theconductivity type in this way, as shown in FIG. 7C, is carried out. Thisstep is carried out by a thermal annealing method using a furnaceannealing oven. In addition, a laser annealing method or a rapid thermalannealing method (RTA method) can be applied. The thermal annealingmethod is carried out in a nitrogen atmosphere having an oxygenconcentration of 1 ppm or less, preferably 0.1 ppm or less and at 400 to700° C., typically 500 to 600° C. In this embodiment, heat treatment at500° C. for 4 hours is carried out. However, in the case where a wiringmaterial used for the second conductive layers 5026 to 5031 is weak toheat, it is preferable that the activation is carried out after aninterlayer insulating film (containing silicon as a main component) isformed to protect the wiring line or the like.

Further, a heat treatment at 300 to 450° C. for 1 to 12 hours is carriedout in an atmosphere containing hydrogen of 3 to 100%, so that a step ofhydrogenating the island-like semiconductor layers is carried out. Thisstep is a step of terminating dangling bonds in the semiconductor layerby thermally excited hydrogen. As another means for hydrogenation,plasma hydrogenation (using hydrogen excited by plasma) may be carriedout.

Then, as shown in FIG. 8A, a first interlayer insulating film 5075 isformed from a silicon oxynitride film into a thickness of 100 to 200 nm.A second interlayer insulating film 5076 formed from an organicinsulating material is formed thereon, and thereafter, contact holes areformed in a first interlayer insulating film 5075, a second interlayerinsulating fihm 5076, and the gate insulating film 5007. Afterpatterning respective wirings (inclusive of connection wiring and signalwiring) 5077 to 5082, 5084, a pixel electrode contacting the connectionwiring 5082 is formed by patterning.

As the second interlayer insulating film 5076, a film made of organicresin is used, and as the organic resin, polyimide, polyamide, acrylic,BCB (benzocyclobutene) or the like can be used. Especially, since thesecond interlayer insulating film 5076 has rather the mearing offlattening, acrylic excellent in flatness is desirable. In thisembodiment, an acrylic film is formed to such a thickness that steppedportions formed by the TFTs can be adequately flattened. It isappropriate that the thickness is preferably made 1 to 5 μm (mostpreferably 2 to 4 μm).

The formation of the contact holes are performed by dry etching or wetetching. Contact holes reaching the n-type impurity regions 5017 and5018 or the p-type impurity regions 5052 to 5074, a contact holereaching to a wiring 5031, a contact hole reaching electric currentsupply line (not shown), and a contact hole (not shown) reaching a gateelectrode are formed, respectively.

Besides, as the wirings (inclusive of connection line and signal line)5077 to 5082, and 5084, a lamination film of three-layer structure isused, in which a Ti film with a thickness of 100 nm, an aluminum filmcontaining Ti with a thickness of 300 nm, and a Ti film with a thicknessof 150 nm are continuously formed by sputtering into one is patternedinto a desired shape. Of course, the other conductive film may be used.

Further, in Embodiment 2, an ITO film with a thickness of 110 nm isformed as a pixel electrode 5083, and then subjected to patterning. Acontact is obtained by arranging the pixel electrode 5083 so as tooverlap with the connection wiring 5082 while contacting therewith.Besides, a transparent conductive film in which 2 to 20% of zinc oxideis mixed with indium oxide may be used. This pixel electrode 5083becomes an anode of an EL element (FIG. 8A).

Then, as shown in FIG. 8B, an insulating film containing silicon(silicon oxide film in Embodiment 2) is formed into a thickness of 500nm, and an opening is formed at a position corresponding to the pixelelectrode 5083 to form the third interlayer insulating film 5085. Uponthe formation of the opening, taper-shape side walls can easily beformed by using a wet etching method. If the side walls of the openingis sufficiently smooth, degradation of the EL layer caused by the stepbecomes a remarkable problem.

Then, an EL layer 5086 and a cathode (MgAg electrode) 5087 arecontinuously formed by vapor deposition without exposing them to theatmosphere. Note that the thickness of the EL layer is preferably set as80 to 200 nm (typically 100 to 120 nm), and the thickness of the cathode5087 is preferably set as 180 to 300 nm (typically 200 to 250 nm).

In this step, the EL layer and the cathode are sequentially formed withrespect to the pixels corresponding to a red color, a green color, and ablue color, respectively. Note that, the EL layer lacks withstandproperty against solutions, and therefore the respective colors must beformed individually without using a photolithography technology. Forthat reason, it is preferred that portions other than desired pixels aremasked using metallic masks, and the EL layer and the cathode areselectively formed only for the necessary portions.

In other words, a mask for masking all the portions except the pixelscorresponding to a red color is first set, and the EL layer emitting ared color and the cathode are selectively formed using the mask. Then, amask for masking all the portions except the pixels corresponding to agreen color is set, and the EL layer emitting a green color and thecathode are selectively formed using the mask. Succeedingly, similarly,a mask for masking all the portions except the pixels corresponding to ablue color is set, and the EL layer emitting a blue color and thecathode are selectively formed using the mask. Note that, in this case,a description is made such that a different mask is used for each case,however, the same mask may be used for all the cases. In addition, it ispreferred that the above process steps are performed while maintaining avacuum state until the EL layers and cathodes are formed with respect toall the pixels.

Employed in this case is a system in which three kinds of EL elementscorresponding to RGB are formed. However, the following systems may beused: a system in which an EL element emitting a white color and a colorfilter are combined; a system in which an EL element emitting a blue orblue-green color and a fluorescing body (fluorescing color conversionlayer: CCM) are combined; and a system in which a transparent electrodeis used for a cathode (opposing electrode) and an EL elementcorresponding to the RGB is overlapped therewith.

Note that known materials may be used for the EL layer 5086. As theknown materials, organic materials are preferably used when taking adriver voltage into an account. For example, a four-layer structureconsisting of a positive hole injection layer, a positive transportationlayer, a light emitting layer, and an electron injection layer may beused as the EL layer. Further, in Embodiment 2, an example is shown inwhich an MgAg electrode is used as the cathode of the EL element,however, the other known material may be used.

Then, a protective electrode 5088 is formed while covering the EL layerand the cathode. A conductive film containing as a main componentaluminum may be used for this protective electrode 5088. The protectiveelectrode 5088 may be formed by vapor deposition using a mask differentfrom that used for forming the EL layer and the cathode, Also, it ispreferred that the protective electrode 5088 is continuously formedafter the formation of the EL layer and the cathode without exposing tothe air.

Finally, a passivation film 5089 made from a silicon nitride film isformed into a thickness of 300 nm. The protective film 5088 actuallyplays a role to protect the EL layer from moisture, etc. However, thereliability of the EL layer may be enhanced by forming the passivationfilm 5089 in addition thereto.

Thus, an active matrix electronic device having a structure shown inFIG. 8B is completed. Note that, in the manufacturing steps of theactive matrix electronic device in accordance with Embodiment 2, asource wiring is formed of Ta or W, which is used for forming the gateelectrode, and a gate wiring is formed of Al, which is a wiring materialused for forming the source and drain electrodes, on account of thecircuit structures and manufacturing steps thereof. However, differentmaterial may be used therefor.

Note that, in the active matrix substrate according to Embodiment 2, byproviding a TFT having a structure which is most suitable not only tothe pixel portion, but also to the driver circuit portion, extremelyhigh reliability can be exhibited and an operational characteristic canbe enhanced. Besides, it is possible to enhance the crystallinity byadding a metallic catalyst such as Ni in the crystallization step. As aresult, a driving frequency of the source signal line driver circuit canbe made 10 MHz or more.

First, in order to prevent the operation speed from lowering as much aspossible, a TFT having a structure capable of lowering a hot carrierinjection is used as an n-channel TFT of CMOS circuit forming the drivercircuit portion. Note that a driver circuit referred to herein includesa shift resistor, a buffer, a level shifter, a latch of a line-sequencedriver, a transmission gate of a point-sequence driver, and the like.

In Embodiment 2, the active layer of the n-channel TFT includes thesource region, drain region, GOLD region, LDD region, and channelforming region, and the GOLD region is overlapped with the gateelectrode via the gate insulating film.

Further, the p-channel TFT of the CMOS circuit does not need to careabout the degradation caused by the hot carrier injection, the LDDregion may not be particularly provided. Of course, the LDD region maybe formed as well as the n-channel TFT to take measures against the hotcarrier.

In addition, in the driver circuit, in the case where such a CMOScircuit that current flow bilateral directions in the channel formingregion, that is, such the CMOS circuit that the role of the sourceregion and the role of the drain region are changed over is used, then-channel TFT forming the CMOS circuit may preferably be formed suchthat the LDD regions are formed on both sides of the channel formingregion so as to sandwich the channel forming region. As the examplesthereof, there are enumerated the transmission gate, etc., used in thepoint-sequence driving. Also, in the driver circuit, in the case wheresuch a CMOS circuit that an off current value must be lowered as much aspossible is used, the n-channel TFT forming the CMOS circuit ispreferably have a structure in which a part of the LDD regions isoverlapped with the gate electrode via the gate insulating film. Thetransmission gate, or the like, which is used for the point-sequencedriving, is also given as the examples thereof.

Note that, in an actual case, if the state shown in FIG. 8B iscompleted, it is preferred that packaging (enclosure) is performed by aprotective film (laminate film, ultraviolet ray curing resin film,etc.), which has a high air-tightness, and is less degassing or atransparent sealing material, in order to protect it from exposing toair. At that time, if the inside of the sealing agent is made into aninert atmosphere, or a hygroscopic material (for example, barium oxide)is arranged the inside thereof, the reliability of the EL elementenhances.

Besides, if the air-tightness is enhanced by the processes such aspackaging, a connector (flexible print circuit: FPC) for connecting theleading terminal and the outside signal terminal is attached thereto, tothereby complete it as a product. In this specification, the electronicdevice refers to the product, which is completed to the state, which canbe shipped.

Further, in accordance with Embodiment 2, the number of photo maskswhich is necessary for the fabrication of the active matrix substratecan be made into five (island-like semiconductor layer pattern, thefirst wiring pattern (gate wiring, island-like source wiring, andcapacitor wiring), a mask pattern for the n-channel region, a contacthole pattern, and the second wiring patterns (inclusive of pixelelectrode and connection electrode). As a result, the manufacturingprocess may be shorten, and it contributes to the lowering of themanufacturing costs, and the improvement of yields.

Embodiment 3

An example of manufacturing an electric device using the presentinvention is explained in embodiment 3.

FIG. 9A is a top view of an electric device using the present invention.FIG. 9B illustrates a cross-sectional view taken along the line X-X′ inFIG. 9A. In FIG. 9A, reference numeral 4001 is a substrate, referencenumeral 4002 is a pixel portion, reference numeral 4003 is a sourcesignal side driver circuit, and reference numeral 4004 is a gate signalside driver circuit. The driver circuits are connected to externalequipment, through an FPC 4008, via wirings 4005 to 4007.

A covering material 4009, an airtight sealing material 4010 and asealing material (also referred to as a housing material) 4011 (shown inFIG. 9B) are formed so as to enclose at least the pixel portion,preferably both the driver circuits and the pixel portion, at thispoint.

Further, FIG. 9B is a cross sectional structure of the electric deviceof the present invention. A driver circuit TFT 4013 (note that a CMOScircuit in which an n-channel TFT and a p-channel TFT are combined isshown in the figure here), a pixel portion TFT 4014 (note that only anEL driver TFT for controlling the current flowing to an EL element isshown here) are formed on a base film 4012 on a substrate 4001. The TFTsmay be formed using a known structure (a top gate structure or a bottomgate structure).

After the driver circuit TFT 4013 and the pixel portion TFT 4014 arecompleted, a pixel electrode 4016 is formed on an interlayer insulatingfilm (leveling film) 4015 made from a resin material. The pixelelectrode 4016 is formed from a transparent conducting film forelectrically connecting to a drain of the pixel portion TFT 4014. Anindium oxide and tin oxide compound (referred to as ITO) or an indiumoxide and zinc oxide compound can be used as the transparent conductingfilm. An insulating film 4017 is formed after forming the pixelelectrode 4016, and an open portion is formed on the pixel electrode4016.

An EL layer 4018 is formed next. The EL layer 4018 may be formed havinga lamination structure, or a single layer structure, by freely combiningknown EL materials (such as a hole injecting layer, a hole transportinglayer, a light emitting layer, an electron transporting layer, and anelectron injecting layer). A known technique may be used to determinewhich structure to use. Further, EL materials exist as low molecularweight materials and high molecular weight (polymer) materials.Evaporation is used when using a low molecular weight material, but itis possible to use easy methods such as spin coating, printing, and inkjet printing when a high molecular weight material is employed.

In this embodiment, the EL layer is formed by evaporation using a shadowmask. Color display becomes possible by forming emitting layers (a redcolor emitting layer, a green color emitting layer, and a blue coloremitting layer), capable of emitting light having different wavelengths,for each pixel using a shadow mask. In addition, methods such as amethod of combining a charge coupled layer (CCM) and color filters, anda method of combining a white color light emitting layer and colorfilters may also be used. Of course, the electric device can also bemade to emit a single color of light.

After forming the EL layer 4018, a cathode 4019 is formed on the ELlayer. It is preferable to remove as much as possible any moisture oroxygen existing in the interface between the cathode 4019 and the ELlayer 4018. It is therefore necessary to use a methods of depositing theEL layer 4018 and the cathode 4019 continually under vacuum or formingthe EL layer 4018 in an inert gas atmosphere and forming the cathode4019 without the air exposure. The above film deposition becomespossible in this embodiment by using a multi-chamber method (clustertool method) film deposition apparatus.

Note that a lamination structure of a LiF (lithium fluoride) film and anAl (aluminum) film is used in this embodiment as the cathode 4019.Specifically, a 1 nm thick LiF (lithium fluoride) film is formed byevaporation on the EL layer 4018, and a 300 nm thick aluminum film isformed on the LiF film. An MgAg electrode, a known cathode material, mayof course also be used. The cathode 4019 is then connected to the wiring4007 in a region denoted by reference numeral 4020. The wiring 4007 is apower source supply line for imparting a predetermined voltage to thecathode 4019, and is connected to the FPC 4008 through a conductingpaste material 4021.

In order to electrically connect the cathode 4019 and the wiring 4007 inthe region denoted by reference numeral 4020, it is necessary to form acontact hole in the interlayer insulating film 4015 and the insulatingfilm 4017. The contact holes may be formed at the time of etching theinterlayer insulating film 4015 (when forming a contact hole for thepixel electrode) and at the time of etching the insulating film 4017(when forming the opening portion before forming the EL layer). Further,when etching the insulating film 4017, etching may be performed all theway to the interlayer insulating film 4015 at one time. A good contacthole can be formed in this case, provided that the interlayer insulatingfilm 4015 and the insulating film 4017 are the same resin material.

A passivation film 4022, a filing material 4023, and the coveringmaterial 4009 are formed covering the surface of the EL element thusmade.

In addition, the sealing material 4011 is formed between the coveringmaterial 4009 and the substrate 4001, so as to surround the EL elementportion, and the alright sealing material (the second sealing material)4010 is formed on the outside of the sealing material 4011.

The filling material 4023 functions as an adhesive for bonding thecovering material 4009 at this point. PVC (polyvinyl chloride), epoxyresin, silicone resin, PVB (polyvinyl butyral), and EVA (ethylene vinylacetate) can be used as the filling material 4023. If a drying agent isformed on the inside of the filling material 4023, then it can continueto maintain a moisture absorbing effect, which is preferable.

Further, spacers may be contained within the filling material 4023. Thespacers may be a powdered substance such as BaO, giving the spacersthemselves the ability to absorb moisture.

When using spacers, the passivation film 4022 can relieve the spacerpressure. Further, a film such as a resin film can be formed separatelyfrom the passivation film 4022 to relieve the spacer pressure.

Furthermore, a glass plate, an aluminum plate, a stainless steel plate,an FRP (fiberglass-reinforced plastic) plate, a PVF (polyvinyl fluoride)film, a Mylar film, a polyester film, and an acrylic film can be used asthe covering material 4009. Note that if PVB or EVA is used as thefilling material 4023, it is preferable to use a sheet with a structurein which several tens of aluminum foil (μm) is sandwiched by a PVF filmor a Mylar film.

However, depending upon the light emission direction from the EL element(the light radiation direction), it is necessary for the coveringmaterial 4009 to have Light transmitting characteristics.

Further, the wiring 4007 is electrically connected to the FPC 4008through a gap between the sealing material 4011 or the airtight sealingmaterial 4010 and the substrate 4001. Note that although an explanationof the wiring 4007 has been made here, the wirings 4005 and 4006 arealso electrically connected to the FPC 4008 by similarly passingunderneath the sealing material 4011 and the airtight sealing material4010.

In this embodiment, the covering material 4009 is bonded after formingthe filling material 4023, and the sealing material 4011 is attached soas to cover the lateral surfaces (exposed surfaces) of the fillingmaterial 4023, but the filing material 4023 may also be formed afterattaching the covering material 4009 and the sealing material 4011. Inthis case, a filling material injection opening is formed through a gapformed by the substrate 4001, the covering material 4009, and thesealing material 4011. The gap is set into a vacuum state (a pressureequal to or less than 10⁻² Torr), and after immersing the injectionopening in the tank holding the filling material, the air pressureoutside of the gap is made higher than the air pressure within the gap,and the filling material fills the gap.

Embodiment 4

A more detailed cross-sectional structure of the pixel portion in anelectric device of the present invention is shown in FIG. 10.

In FIG. 10, a switching TFT 4502 provided on a substrate 4501 is formedby using a p-channel type TFT manufactured by a known method. In thisembodiment, the TFT 4502 has a double-gate structure. Since there is nosubstantial difference in its structure and production process, itsdescription will be omitted. In this embodiment, the TFT 4502 has adouble-gate structure. Due to the double-gate structure, there is anadvantage in that substantially two TFTs are connected in series toreduce an OFF current value. In this embodiment, the TFT 4502 has adouble-gate structure; however, it may have a single gate structure, atriple gate structure, or a multi-gate structure having more gates.

An EL driver TFT 4503 is formed by using the n-channel TFT manufacturedby a known method. A drain wiring 4504 (not shown in the figure) of theswitching TFT 4502 is electrically connected to a gate electrode 4506 ofthe EL driver TFT 4503.

Furthermore, in this embodiment, the EL driver TFT 4503 has a singlegate structure. However, it may have a multi-gate structure in which aplurality of TFTs are connected in series. Furthermore, it may also bepossible that a plurality of TFTs are connected in parallel tosubstantially divide a channel formation region into a plurality ofparts, so as to conduct highly efficient heat release. Such a structureis effective for preventing degradation due to heat.

A wiring (not shown in the figure) including the gate electrode 4506 ofthe EL driver TFT 4503 overlaps a drain wiring 4512 of the EL driver TFT4503 via an insulating film in a part of the region. In the region, acapacitor is formed. The capacitor functions for holding a voltageapplied to a gate electrode 4506 of the EL driver TFT 4503.

A first interlayer insulating film 4514 on the switching TFT 4502 andthe EL driver TFT 4503, and the second interlayer insulating film 4515made of resin insulating film is formed on the first interlayerinsulating film 4514.

Reference numeral 4517 denotes a pixel electrode (cathode of an ELelement) that is made of a conductive film with high reflectivity and iselectrically connected to the drain of the EL driver TFT 4503. As thepixel electrode 4517, a low resistant conductive film such as analuminum alloy film, a copper alloy film, and a silver alloy film, or alayered structure thereof can be preferably used. Needless to say, alayered structure with other conductive films may also be used.

An organic resin film 4516 is formed on the pixel electrode 4517 and theEL layer 4519 is formed after the patterning the portion of facing thepixel electrode 4517. Herein, not shown in the figure, light-emittinglayers corresponding to each color R (red), G (green), and B (blue) maybe formed. As an organic EL material for the light-emitting layer, aπ-conjugate polymer material is used. Examples of the polymer materialinclude polyparaphenylene vinylene (PPV), polyvinyl carbazole (PVK), andpolyfluorene.

There are various types of PPV organic EL materials. For example,materials as described in “H. Shenk, H. Becker, O. Gelsen, E. Kluge, W.Kreuder and H. Spreitzer, “Polymers for Light Emitting Diodes”, EuroDisplay, Proceedings, 1999, pp. 33-37” and Japanese Laid-OpenPublication No. 10-92576 can be used.

More specifically, as a light-emitting layer emitting red light,cyanopolyphenylene vinylene may be used. As a light-emitting layeremitting green light, polyphenylene vinylene may be used. As alight-emitting layer emitting blue light, polyphenylene vinylene orpolyalkyl phenylene may be used. The film thickness may be prescribed tobe 30 to 150 nm (preferably 40 to 100 nm).

The above-mentioned organic EL materials are merely examples for use asa light-emitting layer. The present invention is not limited thereto. Alight-emitting layer, a electric charge transporting layer, or aelectric charge injection layer may be appropriately combined to form anEL layer (for light emitting and moving carriers therefore).

For example, in this embodiment, the case where a polymer material isused for the light-emitting layer has been described. However, a lowmolecular-weight organic EL material may be used. Furthermore, aninorganic material such as silicon carbide can also be used for aelectric charge transporting layer and a electric charge injectionlayer. As these organic EL materials and inorganic materials, knownmaterials can be used.

When the anode 4523 is formed, the EL element 4510 is completed. The ELelement 4510 refers to a capacitor composed of the pixel electrode(cathode) 4517, the light-emitting layer 4519, the hole injection layer4522, and the anode 4523.

In this embodiment, a passivation film 4524 is further formed on theanode 4523. As the passivation film 4524, a silicon nitride film or asilicon nitride oxide film is preferably used. The purpose of thepassivation film 4524 is to prevent the EL element from being exposed tothe outside. That is, the passivation film 4524 protects an organic ELmaterial from degradation due to oxidation, and suppresses the releaseof gas from the organic EL material. Because of this, the reliability ofthe electric device is enhanced.

As described above, the electric device has a pixel portion made of apixel with a structure as shown in FIG. 10, and includes a switching TFThaving a sufficiently low OFF current value and an EL driving TFT isstrong to the injection of hot carriers. Thus, an electric device isobtained, which has high reliability and is capable of displaying asatisfactory image.

In this embodiment, light generated by the light-emitting layer 4519 isirradiated toward reverse direction to the substrate on which a TFT isformed as represented by an arrow.

Embodiment 5

A structure in which the structure of the EL element 4510 in the pixelportion shown in FIG. 10 in embodiment 4 is inverted is explained inembodiment 5. FIG. 11 is used in the explanation. Note that the onlypoints of difference between the structure of FIG. 11 and that of FIG.10 is an EL element portion and an EL driver TFT, and therefore anexplanation of other portions is omitted.

The switching TFT 4502 is formed using a p-channel type TFT manufacturedby a known method in FIG. 11. The EL driver TFT 4503 is formed using ap-channel type TFT manufactured by a known method.

A transparent conducting film is used as a pixel electrode (anode) 4525in embodiment 5. Specifically, a conducting film made from a compound ofindium oxide and zinc oxide is used. Of course, a conducting film madefrom a compound of indium oxide and tin oxide may also be used.

After then forming the third interlayer insulating film 4526, a lightemitting layer 4528 is formed. An electron injecting layer 4529 isformed on the light emitting layer from potassium acetylacetonate(denoted acacK), and a cathode 4530 is formed from an aluminum alloy.

After that, as same as embodiment 5, the passivation film 4532 is formedfor protecting from deterioration by an oxidation. An EL element 4531 isthus formed.

In the case that the EL element has the structure explained in thisembodiment, the light generated by the light emitting layer 4528 isradiated toward the substrate on which the TFT is formed in embodiment5, as shown by the arrows.

Embodiment 6

The electronic devices shown in Embodiments 4 and 5 can be easilyproduced even if reverse stagger type TFTs are used for the TFTsstructuring the driver circuit. An explanation is made here withreference to FIG. 12. Note that locations common with Embodiment 4 andEmbodiment S have reference numerals attached which are the same asthose of FIG. 10 and FIG. 11.

A p-channel TFT formed by a known method is used for a switching TFT4502 formed on a substrate 4501 in FIG. 12. A single gate structure isused in Embodiment 6, but a double gate structure may also be used, anda multi-gate structure, such as a triple gate structure, having three ormore gates may also be used.

Further, a p-channel TFT formed by a known method is used for an ELdriver TFT 4503. A drain wiring 4533 of the switching TFT 4502 iselectrically connected to a gate electrode 4534 of the EL driver TFT4503 by a wiring (not shown in the figure).

Furthermore, although a single gate structure is shown in the figure forthe EL driver TFT 4503, a multi-gate structure in which a plurality ofTFTs are connected in series may also be used. In addition, a structurein which a plurality of TFTs are connected in parallel, effectivelydividing a channel forming region into a plurality of channel formingregions, and which performs heat radiation at high efficiency, may alsobe used. This type of structure is effective as a measure againstdeterioration due to heat.

A wiring (not shown in the figure) containing the gate electrode 4534 ofthe EL driver TFT 4503 overlays with a portion of a source wiring 4535of the EL driver TFT 4503 through an insulating film, and a storagecapacitor is formed in that region. The storage capacitor has a functionof storing a voltage applied to the gate electrode 4534 of the EL driverTFT 4503.

A first interlayer insulating film 4536 is formed on the switching TFT4502 and on the EL driver TFT 4503, and a second interlayer insulatingfilm 4537 made of a resin insulating film is formed on the firstinterlayer insulating film 4536.

Then, similar to Embodiment 5, a pixel electrode (anode) 4538, a lightemitting layer 4539, an electron injecting layer 4540, a cathode 4541,and a passivation film 4542 are formed, forming an EL element 4531.

Light emitted by the light emitting layer 4539 is irradiated toward thesubstrate on which the TFTs are formed, as shown by the arrow, when theEL element has the structure explained in Embodiment 6.

Embodiment 7

In an electronic device having the structure shown in Embodiment 4,light from a light emitting layer 4519 is irradiated in a directionopposite to the active matrix substrate on which the TFTs are formed, asshown by the arrow in FIG. 10. It therefore becomes possible to take avery wide surface area for the light emitting portion because theemitted light is not blocked by TFTs and the like. A structure like thatshown in FIGS. 18A and 18B may be used when one wants to have a pixelportion structure like that shown in FIG. 10. This structure isexplained in Embodiment 7.

FIG. 18A is an example of an overall circuit structure of an electronicdevice shown in Embodiment 7. A pixel portion is arranged in the center.A source signal line driver circuit is arranged on the top side of thepixel portion in order to control source signal lines. To the left sideof the pixel portion is arranged a gate signal line driver circuit inorder to control gate signal lines. A reset signal line driver circuitis arranged on the right side of the pixel portion in order to controlreset signal lines. A portion surrounded by a dotted line frame 1800 inthe pixel portion is one pixel portion circuit, and an enlargementdiagram is shown in FIG. 18B.

The fact that n-channel TFTs are used in a switching TFT 1801 and an ELdriver TFT 1802, and the structure of an EL element 1803, differ fromthe circuit shown in Embodiment 1. The EL element 1803 is formed inaccordance with the structure shown in FIG. 10 of Embodiment 4, andtherefore reference numeral 1810 denotes a cathode, reference numeral1811 denotes an anode, and reference numeral 1809 denotes an anodewiring.

An n-channel TFT is used in the switching TFT 1801 in FIGS. 18A and 18B.The reason for this is explained below.

When a reset TFT 1805 is placed in a conducting state in a certain rowof pixels, write in operation to the pixels has already been completed,and therefore the switching TFT 1801 is in a non-conducting state.Further, there are cases at that point in which the switching TFTs 1801of other rows are conductive, and write in of a signal is beingperformed. In order to set the EL driver TFT 1802 into a non-conductingstate with certainty in a non-display period for a case in which thethreshold voltage of the EL driver TFT 1802 is shifted to a negativevalue, the electric potential of a gate signal line 1806 must be setlower than the electric potential of an electric current supply line1808 by the amount of the threshold voltage of the EL driver TFT 1802while the reset TFT 1805 is in a conducting state. If a p-channel TFT isused for the switching TFT 1801 at this point, then the switching TFT1801 is placed in a conducting state for a case in which the absolutevalue of the voltage between the gate signal line 1806 and the electriccurrent supply line 1808 becomes higher than the absolute value of thethreshold voltage of the switching TFT 1801, due to a drop in theelectric potential of the gate signal line 1806. An n-channel TFT isthus used in the switching TFT 1801 for the pixel shown in FIGS. 18A and18B.

Embodiment 8

In the present invention, a reset signal line driver circuit forcontrolling the operation of a reset TFT is arranged in an independentcircuit with the structure of the example of Embodiment 1, but a singlecircuit structure may also be used as shown in FIG. 19A It is thenpreferable to arrange the gate signal line driver circuit on both sidesof the pixel portion, considering driving. Consequently, as shown inFIG. 19B, the gate signal line driver circuit and the reset signal linedriver circuit may be structured as one circuit, and in addition, thismay be arranged on both sides of the pixel portion.

Embodiment 9

It is possible to easily apply the present invention to an electronicdevice for performing color display of the three colors R (red), G(green), and B (blue). An example for implementation is explained below.As shown in Embodiment 7, a structure in which an n-channel TFT is usedfor an EL driver TFT may be adopted, but in Embodiment 9, a case inwhich a p-channel TFT is used for an EL driver TFT is discussed as anexample, as shown in Embodiment 1.

Brightness characteristics of each color, R (red), G (green), and B(blue) differ in an EL element. Namely, the brightness differs if thesame voltage is applied to am EL element having differing colors ofemitted light. Consequently, there are cases in which the voltageapplied to the EL elements is changed for each color in order to achieveidentical brightness with the three RGB colors. It is necessary toadjust the electric potential of the electric current supply lines ofeach column to voltages adjusted for each color.

When the electronic device and the method of driving an electronicdevice of the present invention are applied to a display such as a colorEL display in which the three RGB colors are separated, the electricpotential of the gate signal lines may be set high with the electricpotential of the electric current supply line to which the highervoltage is applied among the three colors as a basis.

However, in that case, the electric potential difference between theelectric current supply lines to which the lowest voltage is appliedamong the three colors, and the gate signal lines, becomes very large.In other words, the gate voltage of the EL driver TFTs connected to theelectric current supply lines to which the lowest voltage is appliedamong the three colors becomes very high, and there are canes in whichthe leaks of the off current of the EL driver TFTs are increased alittle in those portions. However, the electric potential difference ofthe electric current supply lines is not extremely large, and thereforethis does not become a problem.

Embodiment 10

In this embodiment, an external light emitting quantum efficiency can beremarkably improved by using an EL material by which phosphorescencefrom a triplet exciton can be employed for emitting a light. As aresult, the power consumption of the EL element can be reduced, thelifetime of the EL element can be elongated and the weight of the ELelement can be lightened.

The following is a report where the external light emitting quantumefficiency is improved by using the triplet exciton Cr. Tsutsui, CAdachi, S. Saito, Photochemical processes in Organized MolecularSystems, ed. K Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437).

The molecular formula of an EL material (coumarin pigment) reported bythe above article is represented as follows.

(M. A Baldo, D. F. O' Brien, Y. You, A. Shoustikov, S. Sibley, M. E.Thompson, S. R. Forrest, Nature 395 (1998) p. 151)

The molecular formula of an EL material (Pt complex) reported by theabove article is represented as follows.

(M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest,Appl. Phys. Lett., 75 (1999) p. 4.)(T. Tsutsui, M.-J. Yang, M. Yahiro, K Nakamura, T. Watanabe, T. Tsuji,Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn, Appl. Phys., 38 (1213, (1999)L1502)

The molecular formula of an EL material (Ir complex) reported by theabove article is represented as follows.

As described above, if phosphorescence from a triplet exciton can be putto practical use, it can realize the external light emitting quantumefficiency three to four times as high as that in the case of usingfluorescence from a singlet exciton in principle. The structureaccording to this embodiment can be freely implemented in combination ofany structures of the first to nineth embodiments.

Embodiment 11

The EL display device, which is an application of electric device andits driving method formed according to the present invention, is a selflight emitting type, therefore compared to a liquid crystal displaydevice, it has excellent visible properties and is broad in an angle ofvisibility. Accordingly, the self-emission device can be applied to adisplay portion in various electric devices. For example, in order toview a TV program or the like on a large-sized screen, the self-emissiondevice in accordance with the present invention can be used as a displayportion of an EL display device having a diagonal size of 30 inches orlarger (typically 40 inches or larger).

The EL display includes all kinds of displays to be used for displayinginformation, such as a display for a personal computer, a display forreceiving a TV broadcasting program, a display for advertisementdisplay. Moreover, the electric device and its driving method inaccordance with the present invention can be used as a display portionof other various electric devices.

As other electric equipments of the present invention there are: a videocamera; a digital camera; a goggle type display (head mounted display);a navigation system; a sound reproduction system (a car audio stereo, anaudio set); a notebook type personal computer; a game apparatus; aportable information terminal (such as a mobile computer, a portabletelephone, a portable game machine, or an electric book); and an imageplayback device equipped with a recording medium (specifically, deviceprovided with a display portion which plays back images in a recordingmedium such as a digital versatile disk player (DVD), and displays theimages). In particular, in the case of the portable informationterminal, use of the self-emission device is preferable, since theportable information terminal that is likely to be viewed from a tilteddirection is often required to have a wide viewing angle. Specificexamples of those electric equipments are shown in FIGS. 20A to 21B.

FIG. 20A shows an EL display containing a casing 3301, a support stand3302, and a display portion 3303. The light emitting device of thepresent invention can be used as the display portion 3303. Such an ELdisplay is a self light emitting type so that a back light is notnecessary. Thus, the display portion can be made thinner than that of aliquid crystal display.

FIG. 20B shows a video camera, and contains a main body 3311, a displayportion 3312, a sound input portion 3313, operation switches 3314, abattery 3315, and an image receiving portion 3316. The electric deviceand its driving method of the present invention can be used as thedisplay portion 3312.

FIG. 20C shows a portion (the right-half piece) of an EL display of headmount type, which includes a main body 3321, signal cables 3322, a headmount band 3323, a display portion 3324, an optical system 3325, adisplay portion 3326, or the Like. The electric device and its drivingmethod of the present invention is applicable to the display portion3326.

FIG. 20D shows an image playback device equipped with a recording medium(specifically, a DVD playback device), and contains a main body 3331, arecording medium (such as a DVD) 3332, operation switches 3333, adisplay portion (a) 3334, and a display portion (b) 3335. The displayportion (a) 3334 is mainly used for displaying image information. Thedisplay portion (b) 3335 is mainly used for displaying characterinformation. The electric device and its driving method of the presentinvention can be used as the display portion (a) 3334 and as the displayportion (b) 3335. Note that the image playback device equipped with therecording medium includes devices such as game machines for home.

FIG. 20E shows a goggle type display device (head mount display), andcontains a main body 3341, a display portion 3342 and an arm portion3343. The electric device and its driving method of the presentinvention is applicable to the display portion 3342.

FIG. 20F is a personal computer, and contains a main body 3351, a casing3352, a display portion 3353, and a keyboard 3354. The electric deviceand its driving method of the present invention is applicable to thedisplay portion 3353.

Note that if the luminance of EL material increases in the future, thenit will become possible to use the light emitting device of the presentinvention in a front type or a rear type projector by expanding andprojecting light containing output image information with a lens or thelike.

Further, the above electric devices display often informationtransmitted through an electric communication circuit such as theInternet and CATV (cable TV), and particularly situations of displayingmoving images is increasing. The response speed of EL materials is sohigh that the above electric devices are good for display of movingimage.

In addition, since the EL display device conserves power in the lightemitting portion, it is preferable to display information so as to makethe light emitting portion as small as possible. Consequently, whenusing the EL display device in a display portion mainly for characterinformation, such as in a portable information terminal, in particular aportable telephone or a sound reproduction device, it is preferable todrive the EL display device so as to form character information by thelight emitting portions while non-light emitting portions are set asbackground.

FIG. 21A shows a portable telephone, and contains a main body 3401, asound output portion 3402, a sound input portion 3403, a display portion3404, operation switches 3405, and an antenna 3406. The light emittingdevice of the present invention can be used as the display portion 3404.Note that by displaying white color characters in a black colorbackground, the display portion 3404 can suppress the power consumptionof the portable telephone.

FIG. 21B shows a sound reproduction device, a car audio stereo in aconcrete term, and contains a main body 3411, a display portion 3412,and operation switches 3413 and 3414. The light emitting device of thepresent invention can be used as the display portion 3412. Further, acar mounting audio stereo is shown in this embodiment, but a portabletype audio playback device or a home type device may also be used. Notethat, by displaying white color characters in a black color background,the display portion 3414 can suppress the power consumption. It isespecially effective to portable sound reproduction device.

In the case of the portable electric device shown in this embodiment,the sensor portion is provided to perceive the external light and thefunction to lower the brightness of display when it is used in the darkarea as a method to lower the power consumption.

As described above, the application range of this invention is extremelywide, and it may be used for electric devices in various fields.Further, the electric device of this embodiment may be obtained byfreely combining the structures of the first to tenth embodiments.

The effect of the present invention is discussed.

The overlap of differing address (write in) periods can be avoided withthe present invention, even for cases having short sustain (turn on)periods which cannot be set with a normal time gray scale method, byforming non-display periods. It therefore becomes possible to increasethe number of gray scales.

In addition, the gate voltage of an EL driver TFT (the electricpotential of a gate electrode with respect to a source region of the ELdriver TFT) can be made into a positive value when the non-displayperiods are formed with a reset TFT in a conducting state by regulatingthe electric potential of a gate signal line. Electric current can thusbe prevented from being supplied to an EL element in accordance with theinput of a reset signal even for a case in which the threshold voltageof the EL driver TFT is shifted to a positive value.

1. (canceled)
 2. An electronic device comprising: a pixel comprising: anelectroluminescent element; a capacitor; a first transistor having agate electrode electrically connected to a first line, one of impurityregions electrically connected to a third line; a second transistorhaving a gate electrode electrically connected to the capacitor, one ofimpurity regions electrically connected to a fourth line, and anotherimpurity regions electrically connected to the electroluminescentelement; and a third transistor having a gate electrode electricallyconnected to a second line, and one of impurity regions electricallyconnected to the capacitor, wherein the first, second and thirdtransistors have a same conductivity type.
 3. An electronic devicecomprising: a pixel comprising: an electroluminescent element; acapacitor; a first transistor having a gate electrode electricallyconnected to a first line, one of impurity regions electricallyconnected to a third line; a second transistor having a gate electrodeelectrically connected to the capacitor, one of impurity regionselectrically connected to a fourth line, and another impurity regionselectrically connected to the electroluminescent element; and a thirdtransistor having a gate electrode electrically connected to a secondline, and one of impurity regions electrically connected to thecapacitor, wherein the first, second and third transistors have a sameconductivity type, wherein an electric potential of the first line islower than the fourth line when the first transistor is in a conductivestate, and wherein an electric potential of the first line is higherthan the fourth line when the first transistor is not in a conductivestate.
 4. An electronic device comprising: a pixel provided over asubstrate, and comprising: an electroluminescent element; a capacitor; afirst transistor having a gate electrode electrically connected to afirst line, one of impurity regions electrically connected to a thirdline; a second transistor having a gate electrode electrically connectedto the capacitor, one of impurity regions electrically connected to afourth line, and another impurity regions electrically connected to theelectroluminescent element; and a third transistor having a gateelectrode electrically connected to a second line, and one of impurityregions electrically connected to the capacitor, and a circuit providedover the substrate, and controlling an electric potential of the firstline, wherein the first, second and third transistors have a sameconductivity type.
 5. An electronic device according to claim 2, whereinanother one of impurity regions of the first transistor is electricallyconnected to the capacitor.
 6. An electronic device according to claim3, wherein another one of impurity regions of the first transistor iselectrically connected to the capacitor.
 7. An electronic deviceaccording to claim 4, wherein another one of impurity regions of thefirst transistor is electrically connected to the capacitor.
 8. Anelectronic device according to claim 2, wherein another one of impurityregions of the third transistor is electrically connected to the firstline.
 9. An electronic device according to claim 3, wherein another oneof impurity regions of the third transistor is electrically connected tothe first line.
 10. An electronic device according to claim 4, whereinanother one of impurity regions of the third transistor is electricallyconnected to the first line.
 11. An electronic device according to claim2, wherein the capacitor is electrically connected to the fourth line.12. An electronic device according to claim 3, wherein the capacitor iselectrically connected to the fourth line.
 13. An electronic deviceaccording to claim 4, wherein the capacitor is electrically connected tothe fourth line.
 14. An electronic device according to claim 2, whereinthe electronic device is a device selected from the group consisting ofan electroluminescence display, a video camera, a head mounted display,a DVD player, a personal computer, a portable telephone and a car audiosystem.
 15. An electronic device according to claim 3, wherein theelectronic device is a device selected from the group consisting of anelectroluminescence display, a video camera, a head mounted display, aDVD player, a personal computer, a portable telephone and a car audiosystem.
 16. An electronic device according to claim 4, wherein theelectronic device is a device selected from the group consisting of anelectroluminescence display, a video camera, a head mounted display, aDVD player, a personal computer, a portable telephone and a car audiosystem.